The most common way to contract Babesia is a bite from an infected tick.

Babesia microti parasites live in the gut of the black-legged or deer tick (Ixodes scapularis). The tick attaches to the body of white-footed mice and other small mammals, transmitting the parasite to the rodents' blood. After the tick has eaten its meal of the animal’s blood, it falls off and waits to be picked up by another animal.

The white-tailed deer is a common carrier of the deer tick. The deer itself is not infected. After falling off the deer, the tick will typically rest on a blade of grass, a low branch, or leaf litter. If you brush up against it, it can attach to your shoe, sock, or other piece of clothing. The tick then climbs upward, seeking a patch of open skin.

You probably won’t feel the tick bite, and you may not even see it. That’s because most human infections are spread during spring and summer by ticks in the nymph stage. During this stage, the ticks are about the size and color of a poppy seed.

Besides a tick bite, this infection can also pass via contaminated blood transfusions or via transmission from an infected pregnant woman to her fetus.

People with no spleen or a weakened immune system are at greater risk. Babesiosis can be a life-threatening condition for these people. Older adults, especially those with other health problems, are also at higher risk.

The same tick that carries the Babesia parasite can also carry the corkscrew-shaped bacteria responsible for Lyme disease.

Studies carried out in areas of southern New England, where Lyme disease is prevalent, found that of people diagnosed with Lyme were also infected with Babesia. Researchers also found that the babesiosis often went undiagnosed.

According to the , most cases of babesiosis occur in New England, New York, New Jersey, Wisconsin, and Minnesota. These are states where Lyme disease is also prevalent, though Lyme is also prevalent elsewhere.

The symptoms of babesiosis are similar to those of Lyme disease. Coinfection with Lyme and Babesia can cause the symptoms of both to be more severe.

In the early stages, Babesia parasites can be detected by examination of a blood sample under a microscope. However, this method is reliable only in the first two weeks of the infection.

If you or your doctor suspects babesiosis, your doctor can do further testing. They may order an indirect fluorescent antibody test (IFA) on the blood sample. Molecular diagnostics, such as polymerase chain reaction (PCR), may also be used on the blood sample.

Babesia is a parasite and won’t respond to antibiotics alone. Treatment requires antiparasitic drugs, such as those used for malaria. Atovaquone plus azithromycin is used to treat most cases and is usually taken for 7 to 10 days.

Clindamycin plus quinine is used in more severe cases. With severe illness, additional supportive measures may be taken.

It’s possible for relapses to occur after treatment. If you have symptoms again, they must be retreated.

Inspect your whole body after spending time outdoors. Have a friend look at your back and the backs of your legs, especially behind your knees.

Take a shower and use a long-handled brush on areas you can’t see.

A tick must attach to your skin before it can transmit the disease. Attaching usually takes some hours after the tick has contacted your skin or clothing. Even if the tick attaches, there is some time before it can transmit the parasite to you. You may have as long as . This gives you time to look for the tick and remove it.